Electrolytes for low series-resistance,aluminum electrolytic capacitors

ABSTRACT

AN AQUEOUS ELECTROLYTE FOR ALUMINUM-ANODE CAPACITORS IS COMPRISED MAINLY OF AN ACID SUCH AS MALEIC ACID, A MALEATE OF AN ALIPHATIC AMINE SUCH AS TRIETHYLAMINE, ORTHOPHOPHORIC ACID, A SOLVENT SUCH AS DIMETHYLFORMAMIDE, AND OPTIONALLY A SMALL QUANTITY OF BORIC ACID. SUCH AN ELECTROLYTE OFFERS HIGH ELECTRICAL CONDUCTIVITY AND FOR ALL PRACTICAL PURPOSES A SUFFICIENTLY HIGH MAXIMUM BUILDUP VOLTAGE, THE CAPACITORS WHICH UTILIZE SUCH ELECTROLYTE BEING USABLE OVER A WIDE TEMPERATURE RANGE.

United States Patent Int. (:1. riol 9/02 US. Cl. 252-62.2 9 Claims ABSTRACT OF THE DISCLOSURE An aqueous electrolyte for aluminum-anode capacitors is comprised mainly of an acid such as maleic acid, a maleate of an aliphatic amine such as triethylamme, orthophosphoric acid, a solvent such as dimethylformamide, and optionally a small quantity of boric acid. Such an electrolyte offers high electrical conductivity and for all practical purposes a sufiiciently high maximum buildup voltage, the capacitors which utilize such electrolyte being usable over a wide temperature range.

CROSS-RELATED APPLICATION This Application is a continuation-in-part of Application Ser. No. 139,236, filed Apr. 30, 1971, which in turn is a continuation of Application Ser. No. 770,433, filed Oct. 24, 1968 and both now abandoned.

BRIEF SUMMARY OF THE INVENTION This invention relates to a very slightly aqueous electrolyte for a capacitor which is to function satisfactorily in practical application where low temperatures, high frequencies and the possibility of high temperatures of at least 85 C. are encountered. This objective can be attained by the use of electrolytes which are highly conductive and completely stable over a temperature range of 55 C. to +125 C.

It is well known that an impregnation electrolyte must possess good anodic oxidation capacity in respect of the metal (e.g. aluminum) used for the positive metallized surface. This capacity can be characterized by two quantities:

1. The voltage build-up rate in a specimen of known area, under given current density and temperature conditions.

2. The maximum build-up voltage in the metal specimen under such conditions.

The first of these quantities is relevant to the faradic efficiency of the anodic oxidation and should therefore be high. .The second quantity sets a limit on the service voltages possible with capacitors impregnated with the electrolyte in question and should therefore also be as high as possible. Unfortunately, experience shows that obtaining high electrical conductivity is not compatible with obtaining a high maximum build-up voltage.

The present invention makes it possible to produce electrolytic capacitors utilizing a very slightly aqueous electrolyte which offers high electrical conductivity and at the same time enables a maximum build-up voltage high enough for all practical purposes to be obtained.

The invention also includes in its scope such electrolytic capacitors with a very slightly aqueous electrolyte, which can be used over a wide temperature range.

In accordance with the present invention, these objectives are achieved by impregnating electrolytic capaci- Patented Sept. 10, 1974 tors having an aluminum anode with an electrolyte resulting from the combination:

1. From about 10'- moles up to the upper solubility limit of a maleate of an aliphatic amine selected from the group consisting of where R R and R are C -C alkyl residues;

2. From about 10- moles up to the upper solubility limit of maleic acid; and

3. From about 10* to 10- moles of orthophosphoric acid; these three compounds being dissolved in one mole of at least one solvent selected from the group consisting of dimethylformamide, dimethylacetamide, 'y-butyr0lactone, N-methylpyrrolidine.

Representative aliphatic amines in the compounds above are dior tri-ethyl or butyl-amines, or mixed alkyl amines such as N-methylbutylamine, the preferred amine being triethylamine.

DETAILED DESCRIPTION The following example illustrates a preferred form of the invention and its use in an electrolytic capacitor having an aluminum anode.

A mixture is made up of 13.7 moles of DMF, 0.87 mole of maleic acid, 0.13 mole of orthophosphoric acid (density 1.71) and 1 mole of triethylamine. The mixture is then heated to 120 C. in order to assist dissolving, and subsequent to cooling a solution is obtained which has an electrical conductivity of about 10 mmho/cm. at 25 C., a pH-value of 6.4 at 25 C. and a build-up capacity (measured in a beaker on smooth 99.99% aluminum at C. and 5 ma./cm. characterized by a slope of 55 v./min. and a maximum voltage of about 150 v.

Capacitors With service voltages of 4 v. and 63 v., with etched aluminum anodes were impregnated with such an electrolyte.

Even with a service voltage of 4 v., the impedance at 20 C. is less than twice the impedance at +20 C. at a frequency of 10 kilocycles per second.

Further, after 3000 hours of accelerated ageing at C. at the service voltage, these capacitors exhibit a variation in capacitance of only about 5%.

In order to better clarify the object of the present invention, three sets of electrolytes were produced and compared from the electrical conductivity standpoint. The first set consisted of a variable quantity of maleic acid dissolved in DMF; the second of a variable quantity of N-methylbutylamine maleate (prepared by slowly dissolving one mole of pure maleic acid in two moles of redistilled N-methylbutylamine) in DMF; and the third consisted of a variable quantity of N-methylbutylamine glallltigte with 27 x 10- moles of maleic acid per mole of The results obtained are given in the three tables below:

TABLE 3 N -methylbutylamine maleate witlli) 1242200 mole/mole of maleic acid pedance. Thus, at 55 C., capacitors with a service voltage of 40 v. showed a change in impedance (measured with respect to the impedance at +25 C.) of only 15% at 100 cycles per second and 45% at 1000 cycles Maleate concentration (10' mole/mole 2.5 7.5 25 36 Electrical conductivity, mmho/cm. at 5 per second. I I

35 Variations are obviously possible without departing from the general context of the invention. Thus, it is possible to add a small quantity of boric acid which may be It Wlll be noted that the electrical conductivity of the regarded as a supplier f water at high temperature malelo acid at mole/mote 1S 10 Such variations are well known to the specialist in the and that that of the maleate at mole/mole art, and for this reason it was thought necessary to group mmho/em', and that the eomblhetlon of the afild together, in the following table, the characteristics of with the salt gives 10.5 mmho/cm. whereas an electrical electmlytes according to this invention conductivity of about 7.2 mmho/cm. would have been N f 1 t 1 V ml 0 anticipated. This unexpected etfect constitutes one of the 15 U o e cc r0 yte g y 5 32 3 characteristics of the present invention and is not contemperfliture ranlge 7 fined to the above example as will be demonstrated later. i l g blll1 d-up vo tage Ab vo 5ts.

A further characteristic relates to the maximum buildrate g 10 up voltage. This is about 85 v. (under the conditions E l d at a specified above) for an electrolyte composed of 36x10" cc nca con uctlvlty mm 7 mole/ mole of maleate and 27 10 mole/mole of maleic a out 2 mm 0 acid. It was then found that the addition Of 13 l0 Under the conditions specified precedingly. mole/mole of Phosphoric acid t fatsed the An examination of the above table shows that these P Voltage o about 150 Wlthout atteetlhg the properties taken as a whole represent significant progress electrical conductivlty by more than about 10%. i h capacitor m There iS next given in tabular form in Table 4 Various goes aying changes may be made in eothPo$1tloh$ of electrolyte ShoWlng eleetrleal oohdue' the compositions hereinbefore formulated without departtivity and build-up voltage values, the first example coring f the Scope f the invention responding to the above. Wh t i l i d i In Table 1 P e h the eleetfleal oohdlletlvlty 1. An electrolyte for electrolytic capacitors having aluof a solutlon of the lndicated acid at a strength of minum anodes consisting essentially of:

27 10- mole per mole of solvent, X represents the (a) maleic i electrical conductivity of a solution of the salt of the b a maleate of N-methylbutylamine, triethylamine or above acid and indicated aliphatic amine at a strength ib h i d f 1 e {f mole of Solvent, 3 represents h (c) phosphoric acid dissolved in a solvent consisting electrical conductivity of a solution of the salt and acld ti ll f di th lfo id th mal i id at the m Strengths, 1 represents the maxtmum bulldand maleate respectively being present in an amount p Voltage of the eohltloh o aeld and amlhe, and 2 between 10- moles up to the upper solubility limit represents the maximum build-up voltage of the foreper mole of solvent, the phosphoric acid being presgoing solution to which 13 10- mole of phosphoric 40 ent in an amount between 10 to 10* moles per acid per mole of solvent has been added. mole of solvent.

TABLE4 X2 X1+X2 X3 mmho/ mmho/ mrnho/ rnmho/ Xa(Xi+X U1 U2 7 l/ l Acid Solvent cm. cm. cm. cm. X1+X7 (percent) volts volts (percent) 0.12 7.05 7.17 10.5 +46 85 150 +76 (1 0.20 11.20 11. 40 11.50 +1 88 108 +23 0.20 6.35 6.55 8.55 +30 52 151 +64 0.20 7.50 7.70 8.07 +5 106 174 +64 0.20 4.42 4.62 6.15 +33 100 170 +70 0. 06 3.85 a. 91 4.03 +26 110 300 +100 0.06 2.15 2.21 2.76 +25 112 155 +38 0.03 0.46 0.49 0. 92 +87 135 170 +26 0.07 4.80 4.87 4. 03 +1 150 +100 N-methylpyrrolidone 0.07 2.65 2. 73 3. 75 +37 104 184 +78 50 mole percent DMF, 0 0.11 6.60 6.71 7.62 +13 05 140 +47 mole percent 'ybutyrolactone. .do 50 mole percent DMF, 50 0.11 3.74 3.85 4. 74 +23 100 150 +50 mole percent 'y-butyro- 0118. do 90 r iiole percent DMF, 10 0.15 10.00 10.15 10.30 +1 90 145 +61 11110101 percent ethylene C0 .d0 90 riiole percent DMF, 10 0.15 5.85 6.00 7.65 +27 100 140 +40 mole percent ethylene glycol. NOTE: DMF=dimethyliormamide; MeBuNH=N-methylbutylamine; Bu N=tributylamine; DMAC=dimethylacetamide; EtsN= triethylaminc.

From Table 4 can be seen the unexpected increase in build-up voltage caused by the addition of phosphoric acid and also the production of acceptably high electrical conductivity values with the use of maleic acid with the aliphatic amines.

It was also observed that overall performance of the electrolyte was best when a small quantity of water was present, although in practice the small quantity present in most industrial products could sufiice. It was established experimentally that the maximum quantity of water should be between about 0.01% and 3% by weight.

Capacitors manufactured according to the present invention can be used within a temperature range of 2. An electrolyte as claimed in claim 1, wherein, per mole of solvent, the maleic acid, N-methylbutylamine and phosphoric acid are present in respective concentrations of 27x10 mole, 36 10 mole and 13 10 mole.

3. An electrolyte as claimed in claim 2, wherein said solvent consists of mol percent dimethylformamide and 10 mol percent ethyleneglycol.

4. An electrolyte as claimed in claim 2, wherein said solvent consists of 50 mol percent dimethylformamide and 50 mol percent -butyrolactone.

5. An electrolyte as claimed in claim 2, wherein said solvent consists of 50 mol percent dimethylformamide C. to C. without unduly large variations in im- 75 and 50 mol percent N-methylpyrrolidine.

6. An electrolyte as claimed in claim 1 containing Water in a concentration of between 0.01 and 3.0% by weight.

7. An electrolyte for electrolytic capacitors having aluminum anodes consisting essentially of maleic acid, triethylamine maleate and phosphoric acid dissolved in dimethylacetamide as a solvent, the maleic acid and triethylamine maleate being present in concentrations between 10* mole up to the upper limit of their solubility in said solvent, the phosphoric acid being present in a concentration between 10- mole to 10* mole, per mole of solvent.

8. An electrolyte as claimed in claim 7, wherein, per mole of dimethylacetamide, said maleic acid, said triethylamine maleate and said phosphoric acid are present in respective concentrations of 27 l0 mole, 36 10- mole, and 13 10 mole.

9. An electrolyte as claimed in claim 7 containing water in a concentration of beween 0.01 and 3.0% by weight.

References Cited 5 UNITED STATES PATENTS 3,293,505 12/1966 Chesnot 252-622 X 3,138,746 6/1964 Burger et al 252-622 X 2,965,690 12/1960 Petersen et al. 252-622 X 3,302,071 1/1967 Stahr 252-622 X 3,510,731 5/1970 Thiem et a1 252 62.2 x

JACK COOPER, Primary Examiner Us. 01. X.R. 317 230 

